A panel assembly may be used to enclose a structure which may define an opening or compartment which is to be covered by the panel. The panel may be configured to withstand a force or load imposed on the panel without failing and/or exceeding a maximum deflection. Panel assemblies are used in vehicles to enclose various structures. For example, door panels configured to withstand bending and deflection forces are used to enclose door structures. In another example, a floor panel may be positioned in a floor, trunk, storage, or cargo space of a vehicle to enclose a compartment used to enclose or store one or more vehicle components, such as a spare tire or vehicle jack, or other vehicle user items, such as luggage, tools, etc. The floor panel should be configured to withstand bending and deflection forces and loads imposed by items which may be placed on the floor panel, which may include vehicle users standing, sitting or kneeling on the floor panel. A floor panel may be used in a vehicle including one or more retractable seats, where the floor panel is configured to cover the seat(s) when the seat(s) are fully retracted into a compartment defined by the floor structure of the vehicle. In this configuration, the panel should be configured to withstand loading and deflection forces imposed on the panel when the seats are in the retracted position, which may include withstanding a minimum knee loading requirement. Panels of non-reinforced twin sheet construction may not resist loading forces or may warp or distort in high temperature operating environments. Floor panels strengthened by adding a matrix or grid of reinforcing materials, and/or by increasing the depth of the panel profile or the thickness of the panel are disadvantaged by volume and weight added to the floor panel, such that the panel assembly presents a weight penalty to the vehicle and/or may be difficult to manipulate by a user, and/or the thick panel structure extends substantially into the vehicle compartment being enclosed to consume a portion of the compartment space, thus limiting the open compartment space available for storage, for example, of the retracted seat(s).